Polyazamacrocycles and their derivatives are nitrogen-containing macrocycles known for their sequestration properties of transition metals and heavy metals. They have applications in a wide variety of fields including treatment of liquids, catalysis or health and more particularly medical imaging.
Among these macrocyclic systems, derivatives of the tridentate 1,4,7 triazacyclononane (tacn) ligand, macrocycle carrying three nitrogen atoms, possess very good complexing properties, making them likely to be useful for many applications. In particular, the capacity of nitrogen-containing derivatives of tacn to stabilise metal ions with a high oxidation state and particularly Manganese ions (Mn (IV)) explains their use in many catalytic processes, as in olefin polymerisation reactions or olefin epoxidation reactions with hydrogen peroxide in water (Sibbons et al., Dalton Trans. 2006, 645-661). In this context, the trimethyl-tacn (metacn) system has been especially studied for its catalytic activity in the form of dinuclear complexes of manganese. Complexes of tacn-Mn(IV) have also been suggested as bleaching agents, particularly in detergents, to replace some oxidants (WO01/64826). Tacn derivatives have also been disclosed as cosmetic agents for hair perm, or as keratin extracting agents (FR2862 217, WO2005/049870).
The interest in tacn derivatives has been continuously growing during the last approximately ten years, particularly due to the advantages of this type of system for sequestration of radiometals for applications in SPECT (Single Photon Emission Computed Tomography) and PET (Positron Emission Tomography) nuclear imaging. These macrocyclic molecules are then used as a bifunctional chelating agent capable firstly of sequestering the radioactive source (In, Ga, Cu, Y, etc.) and also of coupling with a vector biomolecule. This indirect labeling method is now a preferred method for medical imaging and for radioimmunotherapy.
NOTA is one tacn derivative frequently used in imaging, and corresponds to the tacn possessing three methylcarboxylate functions on nitrogen atoms. Different physicochemical and structural studies have been done on NOTA complexes intended for SPECT/PET imaging or therapy and in particular have demonstrated that the 64Cu-NOTA and 67/68Ga-NOTA metallic complexes have good in vivo stability. The radiolabeling rate is also relatively fast, requiring mild radiometallation conditions compatible with labeling of antibody type biological molecules (see for example US2012/064003, WO2009/079024, US2011/0070157).
In order to improve existing systems, efforts have been concentrated on optimization of their properties, particularly by developing C-functionalized macrocycles. C-functionalization is an approach that consists of introducing a “grafting” function onto the carbon skeleton of the macrocycle. The C-functionalization of tacn may optionally be accompanied by the addition of chelating arms on the three nitrogen atoms of the cycle. This approach has been applied for the synthesis of new bifunctional chelating agents. However it is relatively limited, because of the difficulty to synthesize C-functionalized macrocycles.
Access to such “bifunctional chelating” systems requires firstly an efficient method of synthesizing the “basic” macrocycle, in this case the tacn motif, and secondly the ability to selectively functionalize this motif at the nitrogen atoms (N-functionalization) and/or in the carbon skeleton (C-functionalization).
To the knowledge of the Applicant, at the present time only one synthesis method is used for the synthesis of 1,4,7-triazacyclononane (tacn) and its N-functionalized analogues, known under the term “Richman-Atkins cyclization” (Richmans, J. E., Atkins, T. J. J. Am. Chem. Soc. 1974, 96, 2268-2270). The method for synthesising tacn described by Richman and Atkins is summarised in the scheme 1 below:

This synthesis method has many disadvantages, particularly the use of tosyl groups during the cyclization step, long reaction times and a global yield of not more than 12%. Drastic conditions for the elimination of tosyl groups (H2SO4, 100° C. for 2 days) form a major obstacle for transposing this method to a large scale, without considering the problem that this synthesis method is not atom-economic.
“Richman-Atkins” cyclization conditions were used for the preparation of C-functionalized derivatives starting from diols or linear amines functionalized on one of the carbon atoms (Argouarch et al., Tetrahedron Lett. 2002, 43, 3795-3798; Argouarch et al., Org. Biomol. Chem. 2003, 1, 2357-2363; Stones et al., Org. Biomol. Chem. 2003, 1, 4408-4417; Scheuermann et al., Org. Biomol. Chem. 2004, 2, 2664-2670; Mc Murry et al., Bioconjugate Chem., 1993, 4, 236-245).
Since the function cannot be introduced into the carbon skeleton by simple C—C coupling, these syntheses make use of biselectrophilic synthons possessing the desired function. This synthesis method is directly inspired from Richman and Atkins' method of obtaining tacn and requires drastic deprotection conditions that considerably limit the nature of functional groups that may be introduced into the macrocycle.
A synthesis method providing access to C-functionalized derivatives was recently described by J. M. Kohlen's team (Koek J. and Kohlen E., Tetrahedron Lett. 2006, 47, 3673-3675). In this method, N-methylated tacn (metacn) is oxidized by N-bromosuccinimide (NBS) to lead to the corresponding bicyclic iminium. The addition of potassium cyanide allows the opening of the bicyclic structure and the formation of a nine-member ring. Reduction of the nitrile group can form an amine function allowing the possibility of further functionalization.

However, this synthesis method requires the preliminary synthesis of metacn by Richman-Atkins cyclization. This method is also very limited because it only provides access to C-functionalized derivatives of metacn. Indeed, methyl groups cannot be removed and therefore, this approach cannot be used for preparation of tacn derivatives possessing coordinating groups on nitrogen atoms (carboxylates, phosphonates, etc.) useful for applications in medical imaging.
Thus, despite the strong potential of C-functionalized tacn systems, very few molecules of this type have been described so far, mainly due to difficulties with synthesis.
Therefore, according to certain embodiments of this invention there is disclosed a new method of synthesis of the 1,4,7-triazacyclononane precursor (tacn) and its N- and/or C-functionalized derivatives. The method according to these and related embodiments comprises the synthesis of imidazo[1,2-a]pyrazin-4-ium salts. Also provided according to certain herein disclosed embodiments are new macrocyclic molecules derived from tacn and imidazo[1,2-a]pyrazin-4-ium salts.